The present invention relates to gas turbine engines, and more particularly to providing a mounting support system for an accessory mounted to a gearbox of the engine which minimizes damage to the accessory and gearbox in the event of a severe rotor imbalance condition such as a fan blade loss.
Modern gas turbine aircraft engines have gearboxes that are mounted on their external cases. The main gearbox is generally positioned under the engine""s core or fan case and uses power from the engine to drive the major accessories needed for engine functions such as generators for electricity, pumps for circulating fluids and heat exchangers for cooling oil or heating fuel. The accessories are frequently mounted on the gearbox.
The main gearbox of modern turbofan engines such as the Model PW4000, manufactured by the Pratt and Whitney Division of United Technologies Corporation, is mounted within the engine core cowl and is driven by an angle gearbox through a towershaft from the high-pressure compressor. The main gearbox drives the fuel pump, scavenge pump, the electrical generator for the electronic engine control, external deoiler, hydraulic pump, and the integrated drive generator (for aircraft electricity).
Various mounting arrangements are used to mount such accessories of the gearbox to the gearbox housing. The accessories, such as the fuel pump are often solidly connected to the gearbox by a series of bolts on a mounting flange at the face of the gearbox.
One problem caused by mounting the gearbox to the engine case is subjecting the gearbox and the associated mounted accessories to high loads caused by rotor imbalances. Severe rotor imbalance can occur in an engine, particularly after a fan blade breaks off from the rotor assembly. One cause of fan blade loss is impact with foreign objects, such as birds, hailstones or other objects which, on occasion, are ingested into the engine. The detached fan blade is thrown outwardly and passes through the fan case, but is typically caught by the fabric wraps in the fan containment case assembly. Blade loss produces an imbalance in the rotor and causes the rotor shaft to deflect radially outwardly. The more the rotor deflects, the greater is the radial load on the rotor bearing supports.
The rotor imbalance loads are transmitted from the bearing supports to the engine case and ultimately to the gearbox fastened to the engine case. The gearbox is forced to move with the engine as the gearbox is fastened to the engine case. In turn, loads are transmitted from the gearbox to the gearbox-mounted accessories. At the interface of the accessory and gearbox, the accessory, due to the relatively high mass thereof, reacts the loads applied to the interface, from the gearbox. Reaction, rather than transmission of such loads through the interface, results in high interface stresses, which may cause cracking at the interface. The cracking may propagate and cause the liberation of the accessories themselves from the gearbox due to the separation at the mounting interface of the gearbox housing and the accessory.
Moreover, if a fuel-handling accessory breaks off from the gearbox or pulls apart, spilled fuel may cause a fire. In particular, the fuel pump and fuel-metering unit mounted to the gearbox, if cracked, may liberate fuel causing such fire conditions. The cracking may initiate at the fuel pump mounting flange which could produce excessive deflections in the interface between the gearbox and the mounted fuel pump which in turn could fracture fuel lines and undesirably release fuel.
Consequently, gearbox housings and accessory mounts have been made thicker, the mounting points stronger, and other strength and durability requirements have been increased. However, these prior art solutions have resulted in substantially heavier hardware which in turn has resulted in the gearbox mount loads to increase. In turn, the efficiency of the engines is adversely impacted by such heavier hardware.
Thus, the challenge for modern gas turbine engines, during severe rotor imbalance events, is the limiting of damage to the accessory mounts, the gearbox and accessories themselves.
A primary object of the present invention is to provide a mounting arrangement for a gearbox accessory, that minimizes the interface stresses between the accessory and the gearbox experienced during a severe rotor imbalance event such as a fan blade loss condition.
According to the present invention, an apparatus for mounting a gearbox accessory for a gas turbine engine substantially isolates the accessory from direct mechanical loading from the engine case during normal engine operating conditions and couples the accessory to the engine case, during high rotor imbalance conditions to minimize interface loads between the gearbox and the accessory. By coupling the accessory to the engine case during a high rotor imbalance condition, the gearbox and the accessory are displaced (vibrate) substantially in unison, thereby minimizing interface loads (stresses) therebetween which would otherwise cause cracking at the interface if reacted by the mass of the accessory.
In the preferred embodiment, the apparatus includes a mounting plate having attachment points for links that extend to and are attached to brackets which are mounted to the engine case. The brackets accommodate bending and torsional deflections of the engine case and accessory and, thus isolating the accessory from the engine case during normal engine operations. The plate has a geometry and thickness sufficient to carry loads between the links and the accessory during a high rotor imbalance condition. A grommet and bushing are disposed at the interface between the links and the plate, which dampen the interaction of the links with the accessory during normal engine operation. During a severe rotor imbalance event, the grommet is compressed and collapses to provide the equivalent of a hard coupling between the accessory and the engine.
A primary advantage of the present invention is the minimization of damage to the accessories, and the gearbox housing during large rotor imbalance conditions, such as in the event of a fan blade loss. The mount for gearbox accessories of the present invention provides acceptable means for minimizing stresses at the interface of the gearbox and accessories. A further advantage of the present invention is its ability to provide an appropriate mounting structure for the gearbox-mounted accessory during both normal operating conditions and a high rotor imbalance event. Another advantage is the ease and cost of manufacturing and incorporating into the mounting arrangement of the prior art, the mount of the present invention. The simplicity of the structure of the mount and the use of economic and readily available materials, allows for cost effective manufacturing processes. The mount of the present invention does not require periodic maintenance and is, thus, cost effective to incorporate into prior art engines. The gearbox housing can be made lighter as the strength of the accessory mounted thereto and the gearbox housing does not have to be increased due to the present invention. As a result, the use of the accessory mount of the present invention is beneficial to the efficiency of the engine.
The foregoing and other objects, features and advantages of the present invention will become more apparent in the following detailed description of the best mode for carrying out the invention and from the accompanying drawings which illustrate an embodiment of the invention.